1. Field of the Invention
The present invention relates to a field effect transistor, particularly to a field effect transistor such as MESFET, HEMT, MISFET or MOSFET.
2. Description of the Related Art
Because a field effect semiconductor device made of compound semiconductor has a high carrier mobility, it is able to operate at a high frequency and used in many fields including supercomputers and microwave communication. It is particularly requested for a field effect transistor used for microwave communication to improve the output, efficiency, and high-frequency operation performance.
A MESFET (metal semiconductor field effect transistor) and HEMT (high electron mobility transistor) are listed as typical field effect transistors having a Schottky electrode among field effect transistors made of compound semiconductor.
FIG. 1 is a sectional view of general MESFET for explaining the prior art.
An i-GaAs buffer layer 2 and a n-GaAs active layer 3 are formed in order on a semi-insulating GaAs substrate 1, on which a source electrode 4, gate electrode 6, and drain electrode 5 are formed separately. The active layer 3 and gate electrode 6 Schottky-contact each other and a depletion layer 7 is formed in the active layer 3 at the joint between the active layer 3 and gate electrode 6. The source electrode 4 and drain electrode 5 ohmic-respectively contact the active layer 3 and a voltage is applied between a source and a drain.
It is well known that in a MESFET the voltage applied to the gate electrode 6 changes the spread of the depletion layer 7 and controls the drain-source current.
The quality of the high-frequency performance is estimated with a value of frequency f which is shown by the following expression. EQU f.sub.1 =g.sub.m /2.pi.C.sub.gs
In the above expression, g.sub.m represents conductance and C.sub.gs represents input capacitance.
As the high-frequency operation performance of a MESFET is better, the cut-off frequency f.sub.1 is higher. To make f.sub.1 higher, it is necessary to decrease the input capacitance C.sub.gs and increase the mutual conductance g.sub.m.
Therefore, to improve the high-frequency operation performance of a MESFET, it is effective to decrease the gate length and C.sub.gs. However, it is described in the following document [1] that the mutual conductance g.sub.m is lower as the gate-length is shorter. The gate-length is a length of the gate electrode in the movement direction of carrier from a source region to a drain region. A direction from the source region to the drain region at the gate electrode is referred to as "gate-length direction".
[1] N. Kato et al., IEEE ELECTRON DEVICE LETTERS, Vol. EDL-4, No. 11, November 1983
FIG. 2 is a sectional view of a MESFET in which the gate length is short to decrease the input capacitance C.sub.gs. In FIG. 2, a symbol same as that in FIG. 1 represents the same element. Generally a short gate length shown in FIG. 2 represents less than 0.5 .mu.m and a long gate length shown in FIG. 1 represents 0.5 .mu.m or more.
The inventor of the present invention performed an experiment for comparing the characteristic of the long gate length of a MESFET with the characteristic of the short gate length of a MESFET. The results are shown in FIG. 3. In FIG. 3, the abscissa represents drain-source voltage V.sub.ds and the ordinate represents drain-source current I.sub.ds.
In FIG. 3, a continuous line represents V.sub.ds -I.sub.ds characteristic of a long-gate MESFET and a broken line represents the characteristic of a short-gate MESFET, and symbol g.sub.mL represents the mutual transfer conductance of the long-gate MESFET and symbol g.sub.ms represents the mutual transfer conductance of the short-gate MESFET. This characteristic diagram uses the gate voltage "Vg" as a parameter.
As the result of comparing the V.sub.ds -I.sub.ds characteristic line of the short-gate MESFET with that of the long-gate MESFET, it is found that the inclination (I.sub.ds /V.sub.ds) of the characteristic line of the short-gate MESFET in the saturated region is larger than that of the long-gate MESFET. Saturation of a characteristic line is caused by the pinch-off phenomenon or carrier speed saturation phenomenon. Increase of drain-source current I.sub.ds in the saturated region represents that current I.sub.1 flowing through the buffer layer 2 is large. The current I.sub.1 flowing through the buffer layer 2 is referred to as "under current".
This is, as shown in FIG. 2, because the depletion layer 7 shortens in the gate-length direction as the gate electrode 8 is shortened and an electric field applied to the both sides of the depletion layer 7 becomes large. The under current I.sub.1 cannot be controlled by a voltage applied to the gate electrode 8.
Therefore, when the under current I.sub.1 increases, the effective mutual conductance g.sub.ms decreases and the cut-off frequency f.sub.1 does not increase as expected. It is described in the above document [I] that a threshold voltage is lowered by extending the interval between the source and drain of the short-gate MESFET.